CONTROLS OF GROUNDWATER-SURFACE WATER EXCHANGE AT NESTED SCALES – WHAT MATTERS WHERE AND WHEN?

Interest in groundwater (GW) surface water (SW) interactions has grown steadily over the last two decades as reflected in the growing number of publications on the topic. The awareness of the significance of GW-SW exchange for water quality and the ecological health of river systems, riparian zones and other groundwater dependent ecosystems has added new scales of investigation to the more regional perspective of classical water quantity management (e.g. conjunctive use of GW and SW). Spatial and temporal patterns of exchange at nested scales and the dynamic processes that control them has become a new area of active research. A better understanding of the dominant controls that create these patterns is important to assess the effects of environmental change on aquatic systems as mandated by new environmental regulations such as the EU Water Framework Directive.

Spatial patterns of exchange are affected by geologic heterogeneities at different scales as well as variations in streambed, channel and landscape morphology. Temporal dynamics are caused by the inherent variability of hydrologic boundary conditions, but also by transitional processes such as the formation of clogging layers or the emergence of aquatic vegetation. Due to the complex interplay between various controls operating at nested space and time scales, it is often difficult to accurately assess local exchange behavior and connectivity between GW and SW in the field. However, such assessments are needed to address questions of nutrient and solute transformations at the GW-SW interface and effects on aquatic and riparian ecology. These complexities are explored using examples from field and modeling studies in different GW-SW systems. Some general ideas are outlined on how to evaluate GW-SW exchange at different scales and for different types of questions.